This invention is a process for producing elemental sulfur and a calcined low-sulfur-content coke from a green high-sulfur-content coke. More particularly, it relates to a staged green coke desulfurization process which includes a first oxidative calcination stage in which a sulfur dioxide-containing off-gas is produced ana a second high-temperature desulfurization-calcination stage in which a hydrogen sulfide-containing off-gas is produced. Yet more particularly, free sulfur is produced by mixing and reacting these off-gases.
Petroleum coke is extensively used in industry, notably as the major constituent of carbon electrodes as packing material in the baking of carbon electrodes and in metallurgy. The greatest proportion of petroleum coke currently available is the product obtained in delayed coking of heavy refinery residual oils and tar. This coke (green coke) is not pure carbon but also contains heavy hydrocarbon compounds and other inorganic impurities. The green coke is unsuitable for the uses mentioned above, and must be calcined. In this step, the hydrocarbons are converted to carbon and volatile matter and the physical structure of the coke is altered.
In the calcination of green coke large volumes of exhaust gases are produced from the calciner. This gas presents a pollution control problem because of the presence of noxious sulfur compounds. This pollution control problem is being compounded by the fact that many of the coker feeds available today have higher sulfer contents then those available in the past. Today, the sulfur content of petroleum cokes often ranges from 4 to 8% sulfur or higher. In general, for industrial purposes, a satisfactory calcined coke usually has a sulfur content below about 2 weight percent and preferably 1 weight percent or lower.
One objective of the process herein is to produce a satisfactory calcined low-sulfur-content coke and at the same time produce a sulfur dioxide-containing gas and a hydrogen sulfide-containing gas which can be used conveniently for other purposes, including the production of elemental sulfur and a residual gas more suitable for venting to the atmosphere. Other objectives will be clear from the following description.
2. Description of the Prior Art
The prior art suggests methods for desulfurizing a green coke by calcining the coke at an elevated temperature. U.S. Pat. No. 2,824,047 shows desulfurizing of a carbonaceous solid using hydrogen gas and an acceptor for hydrogen sulfide which is converted to sulfur dioxide when the acceptor is regenerated.
U.S. Pat. No. 2,734,853 discloses a multistage coke calcining process in which air or a free oxygen-containing gas is employed.
U.S. Pat. No. 3,369,871 shows a three-stage process for the removal of sulfur from a green coke in which the coke is first heated with oxidizing gases to 600.degree.-900.degree.F, then heated to 1600.degree.C (2910.degree.F), and then heated with oxidizing gases to 600.degree.-900.degree.F.
U.S. Pat. No. 3,723,291 shows desulfurizing a green coke which has been impregnated with an alkali metal carbonate. The impregnated coke is heated in the presence of hydrogen gas to a temperature of at least 1000.degree. up to about 2000.degree.F. Evolved hydrogen sulfide is separated from the off-gas by scrubbing.
U.S. Pat. No. 2,721,169 discloses a multistage coke calcining process in which the coke is first subjected to a low-temperature oxidation from 650.degree.to 1000.degree.F followed by a second stage hydrogen gas treatment at 1200.degree. to 1700.degree.F.
The prior art also teaches in a Russian article, "Otraslevaia Konferentsiia po pererabotke vysokosernistykh neftei Materialy," 1st, Ufa, 1965, pp. 119-126; 137-138; 224-234, that sulfur can be removed from coke principally in the form of H.sub.2 S by heating coke in an electrothermal resistance furnace.
The prior art contains information on the effects of certain process variables in the desulfurization of petroleum coke as disclosed in the papers:
1. "Desulfurization of Petroleum Coke During Calcination," by Franc Sef, Industrial & Engineering Chem., Vol. 52, No. 7, July 1960, Pages 599-600; and PA1 2. "Desulfurization of Petroleum Coke," by S. J. Lukasiewiez and G. C. Johnson, Industrial & Engineering Chem., Vol. 52, No. 8, August 1960, Pages 675-677. PA1 a. producing a sulfur dioxide-containing off-gas and a partially desulfurized coke by calcining said green coke under oxidative calcining conditions including a temperature in the range of 1000.degree. to 3000.degree.F; PA1 b. producing a hydrogen sulfide-containing off-gas and said low-sulfur-content coke and continuing the calcination of said partially desulfurized coke by heating said coke under desulfurization reducing conditions including a temperature in the range 2000.degree. to 4000.degree.F; and PA1 c. forming elemental sulfur and water by combining and reacting at least a portion of said sulfur dioxide-containing off-gas with at least a portion of said hydrogen sulfide-containing off-gas. PA1 a. a temperature in the range 1000.degree. to 3000.degree.F, and more preferably a temperature from 2000.degree. to 2500.degree.F; PA1 b. a calcination period in the range 0.1 to 4 hours or more; and PA1 c. the introduction of at least sufficient molecular oxygen into the calciner for the substantially complete oxidation of sulfur in the off-gas to sulfur dioxide. PA1 a. use of a temperature in the range 2000.degree. to 4000.degree.F, and more preferably a temperature from 2500.degree.to 3000.degree.F; PA1 b. a calcination period in the range 0.1 to 4 or more hours; and PA1 c. maintaining a reducing atmosphere in the reaction zone by substantially excluding excess molecular oxygen from the reaction zone, i.e., less than 5 volume percent of the evolved hydrogen sulfide is oxidized.